My cable is sealed like yours and any amount of lubrication, at best, would be a temporary fix if any even got in there. If I were you, I'd replace the cable. It is the least expensive component of the cruise system, other than the speed sensor, and can certainly cause the symptoms you describe, esp. if you are noticing a correlation. If it doesn't work, you could send your amp to Peter at GDL and have it bench tested.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick